542 



Abstract.— The frequency with which 

 dolphins from the northeastern offshore 

 stock of the pantropical spotted dol- 

 phin, Stenella attenuata, experience 

 chase and capture by tuna purse-sein- 

 ers in the eastern topical Pacific Ocean 

 was estimated by comparing dolphin 

 school-size frequencies in sighting data 

 from research vessel observer records, 

 with those recorded in set data by tuna 

 vessel observers. The objective of the 

 S study was to provide a preliminary ba- 

 sis for estimating stock-wide effects of 

 fishery-induced disturbance in these 

 dolphins. 



Our analyses indicate two major 

 characteristics for this stock: first, cap- 

 ture frequency appears to increase rap- 

 idly with increasing school size, and 

 second, approximately half of the stock 

 at any given time occurs in schools 

 smaller than those apparently pre- 

 ferred by purse-seiners. This implies 

 that if individual dolphins have a pref- 

 erence for associating with schools of a 

 particular size, then individuals asso- 

 ciating primarily with large schools 

 would be subjected to chase and cap- 

 ture much more frequently than those 

 associating with small schools. How- 

 ever, because the largest schools are 

 relatively rare and account for a small 

 percentage of individuals, the majority 

 of dolphins in the stock would experi- 

 ence relatively few captures per year, 

 although some would experience a high 

 rate. It is not known whether dolphins 

 do indeed exhibit such a preference, or 

 if instead individuals associate with 

 schools from a wide range of sizes at 

 different times. 



Schools of 1000 or more dolphins are 

 estimated to be set on approximately 

 once a week each on average, but such 

 schools are estimated to represent just 

 under one tenth of the animals in the 

 northeastern offshore stock. Schools set 

 on most often by tuna purse-seiners, 

 containing from about 250 to 500 dol- 

 phins, are estimated to be set on be- 

 tween two and eight times each per 

 year and are estimated to include ap- 

 proximately one third of the stock. An 

 estimated one half of the stock occurs 

 in schools smaller than 250 animals; 

 schools of this size are estimated to be 

 set on less than twice per year each. 



Capture rate as a function of school size 

 in pantropical spotted dolphins, 

 Stenella attenuata, in the 

 eastern tropical Pacific Ocean 



Peter C. Perkins 



Elizabeth F. Edwards 



Southwest Fisheries Science Center 



8604 La Jolla Shores Dr 



La Jolla, California 92038 



E-mail address (for P C Perkins) peter gcaliban ucsd edu 



Manuscript accepted 20 July 1998. 

 Fish. Bull. 97:542-554 (1999). 



Tuna fishermen in the eastern tropi- 

 cal Pacific Ocean (ETP) commonly 

 catch large yellowfin tuna ( Thunnus 

 albacares) by locating a school of 

 dolphins visually and then sur- 

 rounding it with a large purse-seine 

 net in order to capture the tuna that 

 are often closely associated with 

 ETP dolphin schools. The dolphins 

 are released from the net and the 

 tuna are then loaded aboard (NRC, 

 1992). This method, known as "fish- 

 ing on dolphin," historically has 

 been a significant cause of dolphin 

 mortality (NRC, 1992) but has also 

 recently been suggested as a signifi- 

 cant cause of fishery-related physi- 

 ological stress in the dolphins in- 

 volved, perhaps to the point of caus- 

 ing unobserved mortality or changes 

 in reproductive success (e.g. Myrick 

 and Perkins, 1995). 



Although it has not been possible 

 to measure physiological stress di- 

 rectly in these dolphins, it is pos- 

 sible to use existing data to estimate 

 how often an animal experiences 

 chase and capture. Capture fre- 

 quency provides at least a rough 

 measure of the amount of fishery- 

 induced disturbance that dolphins 

 affected by the ETP tuna fishery 

 experience. Here, we estimate cap- 

 ture frequency for the northeastern 

 offshore stock of the pantropical 

 spotted dolphin {Stenella attenuata ) 

 (Dizon et al., 1992). This is the spe- 

 cies most commonly associated with 



tuna and historically most often 

 used in fishing on dolphin (greater 

 than 70*7^ of dolphin sets annually 

 for about the last 30 years (e.g. 

 lATTCM. 



A simple calculation (see "Discus- 

 sion" section) leads to a rough esti- 

 mate for the mean number of times 

 an individual dolphin is set on per 

 year of (number of dolphins set on) 

 H- (number of dolphins ) = 8 times per 

 year. However, simply knowing the 

 overall average rate of capture is 

 not sufficient to evaluate the poten- 

 tial adverse effects on individuals 

 because the rate for different ani- 

 mals may vary widely, depending on 

 a number of interrelated factors in- 

 cluding school size, geographic lo- 

 cation, time of year, and the amount 

 of tuna associated with a school. In 

 this paper, we investigate the effects 

 of school size. Specifically, we show 

 that large dolphin schools (more 

 than several hundred animals) are 

 much more likely to be captured 

 than are small schools (less than 

 one hundred animals) because of a 

 tendency for fishermen to concen- 

 trate their effort on larger schools, 

 which tend to carry more tuna, and 

 to virtually ignore smaller ones. 

 However, this result does not di- 

 rectly give the capture rate for an 



lATTC. 1992. Annual report: 1990. In- 

 ter-American Tropical Tuna Commission, 

 La Jolla, CA, 261 p. 



